The sucrose molecule contains three primary hydroxyl groups and five secondary hydroxyl groups. Therefore, when it is desired to prepare derivatives of sucrose involving reaction of the hydroxyl groups, it can be a major synthesis problem to direct the reaction only to the desired hydroxyl groups. For instance, the artificial sweetener 4,1',6'-trichloro-4,1',6'-trideoxygalacto-sucrose ("sucralose") is derived from sucrose by replacing the hydroxyls in the 4, 1', and 6' positions with chlorine. (In the process of making the sweetener, the stereo configuration at the 4 position is reversed - hence the compound is a galactosucrose.) This compound and methods for synthesizing it are disclosed in U.S. Pat. Nos. 4,343,934, 4,362,869, 4,380,476 and 4,435,440. The direction of the chlorine atoms to only the desired positions is a major synthesis problem, especially since the hydroxyls that are replaced are of differing reactivity (two are primary and one is secondary; the synthesis is further complicated by the fact that the primary hydroxyl in the 6 position is unsubstituted in the final product). The preparation of this sweetener is only one illustration of the synthesis of sucrose derivatives wherein it is desired either to derivatize certain specific hydroxyl groups, and only such hydroxyl groups, or to derivatize only a specified number of the hydroxyls, perhaps in this latter case without particular regard to which particular hydroxyl(s) are derivatized. The preparation of sucrose-based monoester surfactants is a commercial example of monosubstitution on the sucrose molecule.
Some of the Applicants herein and their colleagues at Noramco, the assignee of this application, have developed useful processes for sucrose-6-ester production utilizing as intermediates certain tin compounds. For instance, the distannoxane-based preparation of sucrose-6-esters was described in Navia, PROCESS FOR SYNTHESIZING SUCROSE DERIVATIVES BY REGIOSELECTIVE REACTION, U.S. patent application Ser. No. 220,641, filed on July 18, 1988 now U.S. Pat. No. 4,950,746, and assigned to the same assignee as this application. Navia disclosed that a suitable di(hydrocarbyl)tin-based species, such as dibutyltin oxide, dioctyltin oxide, dibutyltin dimethoxide, or the like, could be combined with a hydroxyl group-containing compound such as a monohydric alcohol or a simple phenol in such a way as to produce a reactive distannoxane intermediate [i.e., a 1,3-di(hydrocarbyloxy)-1,1,3,3-tetra(hydrocarbyl)distannoxane], which could then be reacted with sucrose to afford a 1,3-di-(6-O-sucrose)-1,1,3,3-tetra(hydrocarbyl)distannoxane. Navia also described the ready preparation of sucrose-6-esters by the treatment of these organotin-sucrose adducts with a suitable acylating agent in an appropriate solvent or solvent mixture.
Another tin-mediated process for producing sucrose-6-esters is described in copending U.S. patent application Ser. No. 512,692, SUCROSE-6-ESTER PRODUCTION PROCESS, filed on the same day as this application by D. S. Neiditch, N. M. Vernon, and R. E. Wingard. The process described by Neiditch et al. comprises reacting sucrose with a di(hydrocarbyl)tin oxide in an inert organic vehicle for a period of time and at a temperature sufficient to produce a 1,3-di-(6-O-sucrose)-1,1,3,3-tetra(hydrocarbyl)distannoxane. In a preferred aspect of the process of Neiditch et al., the 1,3-di-(6-O-sucrose)-1,1,3,3-tetra(hydrocarbyl)distannoxane thus produced is reacted with an acylating agent at a temperature and for a period of time sufficient to produce a sucrose-6-ester.
A further tin-mediated process for the production of sucrose-6-esters is described in U.S. patent application Ser. No. 499,731, for SELECTIVE 6-ACYLATION OF SUCROSE MEDIATED BY CYCLIC ADDUCTS OF DIALKYLTIN OXIDES AND DIOLS, filed by R. E. Walkup, N. M. Vernon, and R. E. Wingard on Mar. 23, 1990, and assigned to the same assignee as this application. The process described by Walkup et al. comprises the steps of:
(a) reacting a di(hydrocarbyl)tin oxide, such as a dialkyltin oxide, with a dihydric alcohol, alkanolamine, or an enolizable .alpha.-hydroxy ketone (i.e., an .alpha.-hydroxy ketone that is capable of enolization to an enediol) in an inert organic reaction vehicle with removal of water by azeotropic distillation and at a temperature and for a period of time sufficient to produce a cyclic adduct of said diaklytin oxide and said dihydric alcohol, alkanolamine, or enolizable .alpha.-hydroxy ketone;
(b) reacting said cyclic adduct product of Step (a) with sucrose in an inert organic reaction vehicle in which sucrose has an appropriate degree of solubility, such as a dipolar aprotic liquid, at a temperature and for a period of time sufficient to produce a 6-O-[dihydrocarbyl(hydroxy- or amino- or oxohydrocarbyl)stannoxyl]sucrose; and
(c) reacting the product of Step (b) with an acylating agent to produce a sucrose-6-ester.
In the Navia process, the Walkup et al. process, and the Neiditch et al. process, the reaction mixture containing the sucrose-6-ester ("S-6-E") also contains as a byproduct a 1,3-diacyloxy-1,1,3,3-tetra(hydrocarbyl)distannoxane or distannoxane diester ("DSDE"). This invention provides a process for recovering the said DSDE and, preferably, recycling the tin compound for further production of S-6-E.